CBQE-986: fix argument in _get_indexer fn

Change-Id: Iba4ea8de968cfddff85fb3761d7f3d16d361ab55
Reviewed-on: http://review.couchbase.org/24175
Tested-by: Iryna Mironava <irynamironava@yandex.ru>
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>